Rhizomucor miehei triglyceride lipase is synthesized as a precursor

A Rhizomucor miehei cDNA library constructed in Escherichia coli was screened with synthetic oligonucleotides designed from knowledge of a partial amino acid sequence of the secreted triglyceride lipase (triacylglycerol acylhydrolase EC 3.1.1.3) from this fungus. Lipase-specific recombinants were isolated and their insert sequenced. Unlike characterized bacterial and mammalian triglyceride lipases, the fungal enzyme is synthesized as a precursor, including a 70 amino acid residue propeptide between the 24 amino acid residues of the signal peptide and the 269 residues of the mature enzyme. The precursor processing mechanism, which involves cleavage between a methionine and a serine residue, is unknown. By sequence comparison with other lipases, a serine residue involved in substrate binding was identified in the fungal lipase. The sequence around this residue is well-conserved among characterized lipases. Conservation of an intron in an isolated cDNA recombinant and immunoprecipitation of in vitro synthesized R. miehei translation products indicates that the expression of the lipase gene might involve inefficient mRNA splicing.